Remove DepKind::CrateMetadata and pre-allocation of DepNodes

Remove much of the special-case handling around crate metadata
dependency tracking by replacing `DepKind::CrateMetadata` and the
pre-allocation of corresponding `DepNodes` with on-demand invocation
of the `crate_hash` query.

compiler/rustc_incremental/src/lib.rs

@@ -17,7 +17,6 @@ mod persist;
 pub use assert_dep_graph::assert_dep_graph;
 pub use persist::copy_cgu_workproduct_to_incr_comp_cache_dir;
 pub use persist::delete_workproduct_files;
-pub use persist::dep_graph_tcx_init;
 pub use persist::finalize_session_directory;
 pub use persist::garbage_collect_session_directories;
 pub use persist::in_incr_comp_dir;